# Thermodynamics Practice 3 - AnneReganWiki

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27 Οκτ 2013 (πριν από 5 χρόνια και 3 μήνες)

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AP Chemistry

Unit 11: Thermodynamics Practice
3

1.

2 NO
(g)

+ O
2
(g)

2 NO
2
(g)

H
°=
-
114.1 kJ,

S
°=
-
146.5 J K
-
1

For the reaction at 25

C, the value of the standard free
-
energy change,

G

, is
-
70.4 kJ.

(a)

Calculate
the value of the equilibrium co
n
stant,
K
eq
, for the reaction at 25

C.

(b)

Indicate whether the value of

G

would b
e
come more negative, less negative, or remain unchanged

2.
AgNO
3
(s)

Ag
+
(aq)

+ NO
3

(aq)

The dissolving of AgNO
3
(s)

in pure water is rep
resented by the equation above.

(a)

Is ∆
G

for the dissolving of AgNO
3
(s)

(b)

Is ∆
S

for the dissolving of AgNO
3
(s)

positive, negativ

(c)

The solubility of AgNO
3
(s)

increases with increas
ing temperature.

(i)

What is the sign of ∆
H

for the dissolving pro

(ii)

o parts (b) and (c) (i)? Ex
plain.

The compound NaI dissolves in pure water according to the equation NaI
(s)

Na
+
(aq)

+ I

(aq)
. Some of the

information in the table of standard reduction potentials given below may be useful in answering the

que
s
tions t
hat follow.

Half
-
reaction

EV)

O
2
(g)

+ 4 H
+

+ 4
e
-

㈠2
2
O
(l)

1.23

I
2
(s)

+ 2
e
-

㈠2

=
〮㔳
=
㈠2
2
O
(l)

+ 2
e
-

H
2
(g)

+ 2 OH

=
J
〮㠳
=

+

+
e
-

(s)

-
2.71

(d)

An electric current is applied to a 1.0
M

NaI solu
tion.

(i)

Write the balanced oxidation

half reaction for the reaction that takes place.

(ii)

Write the balanced reduction half
-
reaction for the reaction that takes place.

(iii)

Which reaction takes place at the anode, the ox
i
da
tion reaction or the reduction reaction?

(iv)

All electrolys
is reactions have the sa
me sign for ∆

. Is the sign positive or negative? Justify

3
.

Answer the following questions that relate to the che
m
istry of nitrogen.

(a)

Two nitrogen atoms combine to form a nitrogen molecule, as represented by the following equ
a
tion.

2 N
(g)


N
2
(g)

Using the table of average bond energies below, determine the enthalpy change, ∆
H
, for the rea
c
tion.

Bond

Average Bond Energy
(kJ mol

1
)

N

k
=
ㄶ1
=
k㵎
=
㐲4
=
k

N

㤵9

(b)

The reaction between nitrogen and hydrogen to form ammonia is repres
ented below.

N
2
(g)

+ 3 H
2
(g)

2 NH
3
(g)

H
˚ =

92.2 kJ

Predict the sign of the standard entropy change, ∆
S

(c)

The value of ∆
G
˚ for the reaction represented in part (b) is negative at low temperatures but pos
i
t
ive at

high temperatures. Explain.

(d)

When N
2
(g)

and H
2
(g)

are placed in a sealed co
n
tainer at a low temperature, no measurable amount of

NH
3
(g)

is produced. Explain.

FULL

below

s
:

1.

(a
)
K
eq

=

2.22

10
12

(b
) less negative
;

G

=

H

T

S

; as temperature increases,

T

S

becomes a larger positive value causing an increase in

G

(less neg
a
tive).

2.

(a)

sign of ∆
G

is (

)
;

Because AgNO
3
(s) is known to be soluble in water, the solution process must

be spontaneous.

(b)

sign of ∆
S

is (+)
; an increase in entropy occurs when a solid becomes aqueous and the products
contain more particles than the reactants.

(c)

(i) sign of ∆
H

= (+
)
; an endothermic process will be favored when the temperature is increased.

(ii) yes
;

G

= ∆
H

T∆
S
, as the temperature in
creases the

T∆S term will increase, keeping ∆
G

negative.

(d)

(i) 2 I

I
2
(s)

+ 2
e
-

(ii) 2 H
2
O
(l)

+ 2
e
-

H
2
(g)

+ 2 OH

(iii) anode = oxidation

(iv) sign ∆

= (+); by definition, an electrolysis is
a non
-
spontaneous process and requires the

input of energy to get it to proceed.

3
.

(a)

a triple bond is formed, an exothermic process

H

=

950 kJ mol

1

(b)

(

);

the 4 molecules of gas are reduced
to 2

(c)

G
˚ = ∆
H
˚

T∆
S
˚;
a negative change in entr
opy means that the
T∆
S

factor is positive. When this factor
exceeds the negative value

of the change in enthalpy,
such as at high temperatures, the value of

G
becomes positive (nonspontaneous).

(d)

at low temperatures, the kinetic energy of the mol
e
cule
s is low and very few molecules have enough
activ
a
tion energy